Field of Invention
The present invention relates to a system and a method for electrohydrodynamic printing of liquid on a substrate.
Description of Related Art
The use of ink jet printers for printing information on a medium is well established. Common techniques comprise printers that emit a continuous stream of fluid drops, as well as printers that emit drops only when the corresponding command for emitting is received, respectively. The former group of printers is generally known as continuous ink-jet printers, and the latter as drop-on-demand ink-jet printers, respectively.
In continuous ink-jet printing a high-pressure pump directs liquid ink from a reservoir to microscopic nozzles, thereby creating a continuous stream of ink droplets. The ink droplets are then subjected to an electrostatic field in order to get charged. The charged droplets then pass through a deflecting field such as to be either printed on the substrate or to continue undeflected and being collected in a gutter for re-use.
In drop-on-demand printing, liquid ink is transferred from a reservoir, such as a nozzle, to a substrate by applying a pressure to the reservoir. Ejection of droplets is commonly performed by ways of pressurizing the liquid ink contained inside the nozzle to a degree that allows overcoming of the surface tension and of the viscosity of the liquid. Additionally, the applied pressure has to be sufficiently large in order to accelerate ejected droplets to a velocity that allows precise deposition of these droplets on the substrate. Each time the pressurizing element is triggered, one droplet of a defined volume is ejected, i.e., the printing occurs according to an all-or-none fashion.
Continuous ink-jet printing methods provide a faster throughput than the drop-on-demand methods. The resolution, however, is generally better for the drop-on-demand techniques. Furthermore, continuous ink-jet printing suffers from higher ink losses.
Some of the major problems related to the drop-on-demand and to the continuous ink-jet printing methods are the high pressures required for the ejection of small droplets (where small refers to a size below a few tens of micrometers) and the difficulty of depositing these small droplets with high accuracy, respectively. Droplets being smaller than 10 micrometers are easily decelerated and deflected by their gaseous environment. Furthermore, the droplets ejected by liquid pressurization are generally equally large or even larger than the nozzle they are ejected from. Therefore, in order to obtain small droplets, small nozzles are required which however, suffer from the well-known problem of getting clogged easily.
Electrohydrodynamic jet printers differ from ink-jet printers in that they use electric fields to create fluid flows for delivering ink to a substrate. Especially, electrohydrodynamic printing enables the printing of droplets at much higher resolution than compared to ink-jet printing. While conventional ink-jet printing employs internal pressure pulses to push liquid out of a nozzle, electrohydrodynamic printing methods make use of the fact that liquid can be electrically charged and be pulled out of a nozzle by the force established between the charged liquid and the electric field that is applied in the region of the nozzle.
WO 2007/064577 A1 discloses a common stimulation electrode, which, in response to an electrical signal, synchronously stimulates all members of a group of fluid jets emitted from corresponding nozzle channels to form a corresponding plurality of continuous streams of drops.
A method for manufacturing a collective transfer ink-jet nozzle plate is disclosed in EP 1844 935 B1, where a three-dimensional structure is arranged on a substrate according to a micro ink-jet printing method which is then covered with a curing material. After curing, micro nozzle holes are formed in the plate of the curing material.
EP 1 550 556 A1 discloses a method for producing an electrostatic liquid jetting head comprising a nozzle plate and a driving method for driving the electrostatic liquid jetting head. When a voltage is applied to a plurality of jetting electrodes arranged on a base plate, droplets are ejected from a plurality of nozzles that are arranged on the electrostatic liquid jetting head.
High-resolution electrohydrodynamic ink-jet printing systems and related methods for printing functional materials on a substrate surface are disclosed in US 2011/0187798, where, e.g., a nozzle is electrically connected to a voltage source that applies an electric charge to the fluid in the nozzle to controllably deposit the printing fluid on the surface, and wherein the nozzle has a small ejection orifice such that nanofeatures or microfeatures can be printed.
A method for the production of 1D, 2D and/or 3D depositions from a liquid loaded with nanoparticles or other solid-phase nano-compounds is disclosed in WO 2013/00558, where a nozzle-ended container holds the liquid, an electrode is in contact with the liquid at the nozzle or in the container, and where a counter electrode is located in and/or on and/or below and/or above a substrate onto which the depositions are to be produced.
Many different ways of droplet ejection are possible in electrohydrodynamic printing, the most common one being cone-jet printing, in which a thin jet is ejected from a much larger nozzle (i.e. a jet with smaller radius compared to the radius of the corresponding nozzle). Electrohydrodynamic liquid ejection has been extensively used in the area of electrospraying and electrospinning, but only recently it has found application in controlled printing. Current applications generally suffer from problems related to the strongly charged nature of the ejected liquid. This often results in repulsion of ejected droplets and as a consequence to variations in their positions of impact on the substrate. Repulsion may either occur between two airborne droplets or between airborne droplets and the charge associated with droplets that are already deposited on the substrate.
Furthermore, very high voltages are often required for causing the ejection of liquid. One of the main issues related to electrohydrodynamic liquid ejection is its requirement for very high electrical fields, which are higher than the dielectric breakdown strength of air.
This issue is generally solved by using sharp nozzles and curved counter electrodes (e.g. ring electrodes) that focus the electric field. However, the electrical fields established between the nozzles and the counter electrodes usually decrease with increasing distance between the nozzles and the counter electrode. The average electric fields established between the nozzles and the counter electrodes are therefore low enough in order to not cause electrical breakdown. However, once a charged droplet is ejected, it has to be accelerated towards the substrate, especially if the droplet is smaller than 10 μm or even smaller than 1 μm, i.e., if the droplet experiences a gravitational acceleration which is negligible. Since electrohydrodynamic printing can generate droplets with diameters being smaller than 100 nm, strong accelerating electric fields are therefore crucial for an accurate placement of the droplets.
Especially the deposition of droplets on dielectric substrates can result in substantial spraying deflection of the approaching charged droplets due to the residual charge of prior droplets already deposited on the substrate. This effect becomes more problematic if the accelerating electric field strengths decrease towards the substrate. In this case, the electric field originating from the charge of already deposited droplets will be stronger than the accelerating fields which might result in repulsion of incoming droplets on the substrate that are equally charged as the already deposited droplets. Of course, repulsion can also take place between airborne droplets if the accelerating fields are not set to compensate the deflection resulting from residual electrical charge on the airborne droplets.
Electrical crosstalk may result from the interaction between closely arranged nozzles and the droplets ejected from these nozzles. A close arrangement and the parallel operation of a multitude of electrohydrodynamic nozzles are also hindered by the fact that these nozzles would have to be operated with very high voltages that are difficult to control.
NanoDrip printing, i.e., the printing of nanoscale droplets, allows a printing resolution of better than 100 nm. If, however, a large area shall be printed at such a high resolution within a reasonable time, the print head would have to be scanned with a velocity in the range of tens of millimeters per second or even meters per second, and the nanoscale droplets could no longer be deposited on the substrate with sufficient accuracy. In addition, in order to deposit droplets within a spacing of about 100 nm at a scan velocity of one meter per second, the droplet ejection would require an ejection frequency of around 10 MHz.
Because the droplets are small in NanoDrip printing, these droplets only cover a very small area on the substrate they are printed on. In order to print a large area on a substrate at industrially relevant throughput, a multitude of densely arranged nozzles is needed compared to ink-jet printing or electrohydrodynamic printing performed at a low resolution, while at the same time cross-talk between such densely arranged nozzles and between the droplets they eject, must be prevented, such that nozzles can be individually addressed and droplets be deposited on a substrate with high accuracy.